Im guessing no one has done this then lol. I tried it anyways. I drilled a hole in the side of the overflow for the emergency, I had to shave off the sides of the bulkhead so it fit in there. I drilled almost right at the top which looked right at first but I didn't account for the tank being slightly higher, in retrospect I would have put it a little lower. When it runs in full emergency the tank is full right to the top and it splashes over a bit, not major but not perfect. It seems to run as a herbie should, but needs readjustment after a water change or a power out. I don't recommend this setup (or HOBs in general) but for an established tank that you don't want to move it seems to be working out.
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